1. Field of the Invention
This invention relates to fluid flow meters and more particularly to flow meters for measuring flow within an oil or gas well environment. Specifically, this environment will generally include exposure to high temperatures, high pressures, corrosive media, shock and vibration. Additional requirements are also a small diametrical size, low power consumption and the ability to make measurements while in motion.
2. Description of Related Art
Fluid flow within an oil or gas well has long been a very important parameter for well troubleshooting and evaluations. The most suitable instrument for this measurement has traditionally been based on a rotating turbine type of flow meter. The rotating impeller does give a good measure of the fluid velocity but it also has limitations imposed by friction and the required application of precision bearings within a hostile, contaminated fluid environment.
The effect of unavoidable friction in the impeller bearings is to limit the lowest measurable flow to that which is necessary to overcome the bearing friction losses. These friction losses can be reduced by making the bearings, and impeller, more sensitive but then this can also make the flow meter more susceptible to the effects of contamination and/or wear.
U.S. Pat. No. 5,463,903 discloses a fluid flow sensor having no moving parts. An impeller element of the assembly is substantially static. Fluid flow over the static impeller imposes a static torque stress upon a non-rotating shaft. This torsional stress is transferred to a variable capacitor, the values of which are calibrated to fluid flow rate values. Although this '503 flow meter has no moving parts and no minimum flow threshold, there is, in actual practice, a lower limit of flow measurement as a result of the exponential relationship between torque and flow:Torque=K·ρ·V2 Where K is a proportionality constant, ρ is the fluid density and V is the flow velocity. Flow rate, of course, is directly proportional to flow velocity so it becomes necessary to take the square root of torque to obtain a linear flow relationship. This square root relationship, however, has the effect of limiting the useable dynamic flow measurement range. For example, if the proportionality constant, K, is chosen so that 100% of full scale (FS) flow is equal to 100% FS of the torque sensor, then 1% FS flow would be equal to 0.01% of the torque sensor full scale. Since errors related to the torque sensor and its environment can easily exceed this value, then a measurement of 1% FS flow would be extremely inaccurate. It is also apparent that any small shift in the zero reading of the overall sensor could easily appear as a flow when none was present.
It is an object of this invention, then, to provide a torque flow sensor suitable for making accurate flow measurements over a wider range of flows, within the conditions encountered in oil or gas wells.